Subscriber systems, such as cable television systems, have been configured to provide programming according to a scheduled or set time table. The channel line up of a cable television system is selected by choosing the carrier frequencies of the base band signals and frequency division multiplexing the various carriers together to produce the spectrum of a particular system. On each channel the programs, usually from one half hour to two hours in duration, are scheduled in time sequence. The program schedules change infrequently, normally about once a year when new series are scheduled to replace other less popular programs. The channel line up changes less frequently, usually when a new programmer (channel) is signed onto a system or an old one is taken off.
If a particularly popular episode of a series is missed by a subscriber, and he has not recorded it on a VCR, he may have to wait until the second half of the season when it is rerun to view it. If a special or one time programming event is missed, the subscriber may never have another opportunity to view the program. Movies are in this category where they may not be repeated as often as a subscriber would like.
On demand service systems have been proposed to overcome many of the difficulties in the lack of selection in programming and scheduling. Video on demand, or near video on demand, systems essentially attempt to provide every subscriber the program he wants to view when he wants to view it. However, a pure program on demand system which would have an infinite library which could be addressed at any time by an infinite number of subscribers remains elusive. The cost and complexity or systems which approach such goals even for the limited number of subscribers in a typical cable television system remain prohibitive.
There have been two advances in technology which have brought these systems closer to realization. The first has been the file server technology which permits the storage and retrieval of massive amounts of digital information very quickly. This has made the storage of large programming libraries in digital format more of a reality. To augment this advance in storage technology there has also been the increased use of compression algorithms, particularly the MPEG (Motion Picture Experts Group) algorithm for video signals. Compression allows even more video programs to be stored in the same digital memory size by removing the redundancies in the signal. A standard video signal such as a NTSC formatted signal which is commonly used in the United States has a great deal of spatial redundancy (areas in a scene or field do not change abruptly in hue, tint, and luminance except at boundaries) and a great deal of time redundancy (scenes or fields do not change abruptly in hue, tint, and luminance except for moving objects). The MPEG algorithm has been optimized to eliminate both spatial and time redundancies in video signals and provides for compression of a standard NTSC signal to the extent that 2-10 compressed digital signals can be carried in the same bandwidth (6 MHz) conventionally reserved for a single analog signal.
There remains the problem of the manner in which the stored signals should be broadcast to the subscribers. If they are sent in a compressed digital form, then each subscriber, in addition to the equipment presently in place, should have a digital demodulator, a decompressor, a digital to analog converter, and a NTSC format amplitude modulator. It is readily apparent that such a configuration may add significant cost to such systems. The digital signals also require different processing for authorization and control than do the analog transmissions conventionally used in most CTV systems.
Premium channels from which system operators receive an increasing part of their revenue create more complexity if the digital transmission method is used because their program denial, authorization and addressing methods are incompatible with that of analog systems.
In the past analog systems, a scrambler has been provided to scramble premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/descrambler at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/descrambler and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the descrambler portion of the converter/descrambler to descramble the scrambled premium channel or program.
One popular method of analog scrambling is sync suppression where the horizontal synchronizing pulses of the video signal are attenuated in level into the range of the active video portion of a signal so that a subscriber receiver can no longer decode them. The suppression can be static and provide one level of suppression for all synchronizing pulses or dynamic where the level of attenuation is changed either on a field by field basis, a line by line basis, or randomly. To decode this type of scrambling the converter/descrambler at the subscriber must be sent information on the timing of the sync pulses and their attenuation level from some reference. Another method of analog scrambling which is used extensively, either alone or in combination with sync suppression, is video inversion where the active video portion of a particular horizontal line is inverted about a reference level or axis. To decode this type of analog scrambling, the converter/descrambler must be sent information on which lines have been inverted and the reference level of the axis of inversion.
What is needed is an on demand services system which can take advantage of the authorization and control protocols, and the spectrum allocation, of the CTV systems presently in place while still exploiting the newer digital technologies of file servers, compression, and digital control and processing.
An advantageous on demand services system which meets these goals is described in U.S. patent application entitled "Method and Apparatus for Providing On Demand Services In A Subscriber System", filed on Oct. 1993 in the names of Farmer, et al. which is assigned commonly with the present application. The disclosure of Farmer, et al. is expressly incorporated herein by reference.
In the referenced on demand services system it was noted that it was advantageous to use a scrambled analog video signal which had been digitally stored in a file server in a digital format. In one preferred embodiment, the system discloses digitizing a scrambled analog signal and compressing the digitized signal with a compression algorithm, such as the MPEG compression algorithm. While the MPEG compression algorithm and other video compression algorithms work relatively well on the active video portion of a video signal, the synchronizing portions of such signals present a different problem. This is particularly true when the synchronizing portions of the signals have been scrambled using the sync suppression and/or video inversion scrambling methods. Moreover, the descrambling information included in many in band scrambling systems is not easily digitized by such compression algorithms.